1. Field of the Invention
This invention relates to laminated grid and webbed magnetic cores and, more particularly, it pertains to high reluctance core legs, such as high voltage shunt reactors or other electromagnetic devices, that require linear magnetization characteristics.
2. Description of the Prior Art
The function of a shunt reactor is to provide the required inductive compensation necessary for line voltage control and stability in high voltage transmission lines. The prime requisites of a reactor are to sustain and manage high voltage (about 700 kV) and to provide a constant inductance over a range of operating inductions. Simultaneously, the reactors are to have low profile in size and weight, low losses, low vibration and noise, and sound structural strength.
Current conventional shunt reactors are constructed in a manner similar to the core type power transformers in that both use high permeability low loss grain oriented electrical steel in the yoke sections of the cores. However, they differ markedly in that shunt reactors must provide constant inductance over a range of operating inductions. In conventional high voltage shunt reactors, this is accomplished by use of a number of large air gaps in the leg sections of the reactor core. Typically, the high reluctance legs consist of approximately one inch of air gap followed alternatively by one inch of electrical steel. In current practice, the iron or ferromagnetic sections of the high reluctance core are constructed by cutting and assembling electrical steel strips into what resembles a multi-spoke wheel. Such sections are difficult to construct because of the requirement to utilize progressively smaller strips as building proceeds from the center to the circumference of the section. The design is complicated further by space factor and bonding strength requirements.
The core legs are constructed by alternating the "wheels" with ceramic spacers to provide the required air gap and to provide an integrated structure. An example of a reactor leg consists of 18" of iron "wheels" followed alternatively by 18" of air gap (ceramic discs). This design has high losses due to leakage flux impinging on the iron at an angle somewhat normal to the plane of the lamination strips. Because of B.sup.2 A forces at the air gaps, high amplitude vibrations produce high noise levels. This structure is difficult to construct and assemble due to the large number of strips that must be stacked on end into the wheel design. Since the structure uses ceramic inserts as spacers for air gaps, this tends to produce a weakened structure.
Another example of conventional shunt reactors is the all air gap reactor. This reactor has the advantage of having perfectly constant inductance and consequently has a constant derivative of voltage with respect to current, i.e., .DELTA.E/.DELTA.I=constant. A marked disadvantage to this design is the low permeability of the reactor, the permeability being equal to that of space which is equal to one gauss/oersted, or unity. This means that for a given inductance this design will by necessity have a size two or three times that of an iron-air gap reactor. In addition, due to the low circuit permeance, stray eddy current losses, particularly in the windings, will be exceedingly high compared to the iron air gap designs.